Method and Apparatus for Monitoring the Autonomous Nervous System of a Sedated Patient

ABSTRACT

The invention relates to a method and an apparatus for monitoring the autonomous nervous system of a sedated patient. According to the method, skin conductance is measured through a time interval. Average skin conductance values and the number of fluctuation peaks are in the interval are calculated and analyzed, and an indication is given of the state of pain/discomfort in the patient as well as the state of awakening in the patient. The invention is particularly applicable for use with anaesthetized or artificially ventilated patents, as separate output signals are automatically provided, indicating the need for analgesics and hypnotics, respectively.

TECHNICAL FIELD

The invention relates in general to medical technology, and inparticular to a method and an apparatus for monitoring the autonomousnervous system of a sedated patient. More specifically, the inventionrelates to a method and an apparatus for concurrently indicating thepossible state of pain/discomfort and the possible state of awakening ofa sedated patient, based on measurements of the patient's skinconductance.

BACKGROUND OF THE INVENTION

In the field of medical technology there is a problem in producingphysical measurements representing the activity in an individual'sautonomous nervous system, i.e. in the part of the nervous system, whichis beyond the control of the will. Particularly, there is a special needto monitor the autonomous nervous system of a sedated, non-verbalpatient, e.g. a patient in anaesthesia or an artificially ventilatedpatient, in order to detect if the patient needs more analgesics due topain/discomfort stimuli or hypnotics because of awakening stimuli.

Analgesics are given to avoid pain/discomfort, and hypnotics are givento avoid awakening. Pain/discomfort can induce awakening and awakeningrarely induce pain/discomfort. If hypnotics is given to a patient thatfeels pain/discomfort, the stress activation may be reduced, but thepatient may still feel pain/discomfort. It is therefore a need forproviding a monitoring system that can give information about if thestress activation found in sedated patients is due to pain/discomfortstimuli or awakening stimuli.

Both patients in anaesthesia and patients that are artificiallyventilated are treated with analgesics and hypnotics. Currently, thestress activation of these patients is monitored by an increase in bloodpressure and heart rate. Blood pressure and heart rate is influenced bymany other factors than the need of analgesics or hypnotics, like bloodcirculatory changes found in heart disease, hypertension, lung disease,anaemia, blood loss and sepsis to name a few.

1-2% of the patients in anaesthesia feel pain during surgery. Thedevelopment in medication in anaesthesia is to give both hypnotics andanalgesics with very short half-life. Then it will be even moreimportant to monitor the patient's need of analgesics and hypnotics.

Tests have shown that the skin's conductance changes as a time variablesignal which, in addition to a basal, slowly varying value (theso-called basal level or the average conductance level through a certaininterval), also has a component consisting of spontaneous waves orfluctuations, in which characteristics of these fluctuations, such asfor example their frequency and amplitude, are factors which arecorrelated with the experience of pain in the target object (thepatient). Measuring and analyzing characteristics of these fluctuationsis a known method of providing information concerning the activity inthe sympathetic nervous system, including the effect of pain.

RELATED BACKGROUND ART

WO 00/72751 A1 discloses a method and an apparatus for monitoring theautonomous nervous system of an individual. According to the method, ameasurement signal is provided, expressing the conductance of at leastone area of the individual's skin. The measured signal values are storedat discrete points of time in a time window. Further according to themethod, an analysis of the measurement signal is performed in the timewindow, including calculating the amplitude and the number offluctuation peaks in the conductance signal in said time window. Fromsaid characteristics of the peaks an output signal is established,indicating the state of pain in the individual.

This method provides an indication of pain, but no indication ofawakening. When used for monitoring a sedated patient, the prior artmethod may provide an indication of lack of analgesics, but it does notprovide an indication of lack of hypnotics due to awakening.

WO 85/00785 A1 discloses an apparatus for monitoring the attention orconcentration of a person such as a long distance coach driver. Volarskin resistance is monitored, and if the resistance rises more than apredetermined amount, a stimulus, e.g. an audible tone in a headset maybe emitted to alert the driver and to increase his concentration.

Although this simple way of monitoring of the skin resistance value maygive an indication of the person's attention, it would not provide anindication, which is satisfactorily reliable for use when monitoring theawakening of a sedated patient. In particular, this prior art solutiondoes not provide indications of pain/discomfort and awakeningsimultaneously.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and anapparatus for monitoring the autonomous nervous system of a sedatedpatient, which indicates a state of pain/discomfort in the patient andwhich also provides an indication of awakening of the patient.

Another object of the invention is to provide such a method andapparatus, which relies on the measurement of skin conductancevariations due to emotional sweating.

Still another object of the invention is to provide such a method andapparatus, which provides reliable output indications.

According to the invention, the above objects are achieved by the methodas indicated in the appended claim 1 and by the apparatus as indicatedin the appended claim 10

Further advantages and characteristics of the invention are indicated inthe dependent claims.

Emotional sweating provides a more accurate and precise means ofmonitoring the need for hypnotics (after awakening stimuli) andanalgesics (after pain/discomfort stimuli) than measurements of bloodpressure and heart rate. As opposed to blood pressure and heart rate,the emotional sweating is not influenced by blood circulatory changesfound in heart disease, hypertension, lung disease, anaemia, blood lossand sepsis, to name a few. Moreover, when measuring blood pressure andheart rate changes, health care staff is not able to know if the patientneeds more analgesics or hypnotics.

BRIEF DESCRIPTION OF THE DRAWINGS

The apparatus and method according to the invention will now bedescribed in more detail with reference to the attached drawings, inwhich

FIG. 1 is block diagram for an apparatus according to the invention, and

FIG. 2 is a flow chart illustrating a method according to the invention,

FIGS. 3 a-c are three graphs, each illustrating a time-series of skinconductance measurements of a sedated patient, which is exposed toneither awakening stimuli nor pain/discomfort stimuli,

FIG. 4 is a graph illustrating a time-series of skin conductancemeasurements of a sedated patient which is exposed to pain/discomfortstimuli, and

FIG. 5 is a graph illustrating a time-series of skin conductancemeasurements of a sedated patient which is exposed to awakening stimuli.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a block diagram for a preferred embodiment of anapparatus according to the invention. The apparatus is particularlyarranged for the simultaneous detection of pain/discomfort reaction andawakening in a sedated patient. On an area 2 of the skin on a body part1 of the patient, sensor means 3 are placed for measuring the skin'sconductance. The body part 1 is preferably a hand or a foot, and thearea 2 of the skin on the body part 1 is preferably the palmar side ofthe hand (in the palm of the hand) or the plantar side of the foot(under the sole of the foot). The sensor means 3 comprise contactelectrodes where at least two electrodes are placed on the skin area 2.In a preferred embodiment the sensor means 3 consist of threeelectrodes: a signal electrode, a measuring electrode and a referencevoltage electrode, which ensures a constant application of voltage overthe stratum corneum (the surface layer of the skin) under the measuringelectrode. The measuring electrode and the signal electrode arepreferably placed on the skin area 2. The reference voltage electrodemay also be placed on the skin area 2, but it is preferably placed in anearby location, suitable for the measuring arrangement concerned.

In a preferred embodiment an alternating current is used for measuringthe skin's conductance. The alternating current advantageously has afrequency in the range of up to 1000 Hz, corresponding to the area wherethe skin's conductance is approximately linear. A frequency should beselected which ensures that the measuring signal is influenced to theleast possible extent by interference from, e.g., the mains frequency.In a preferred embodiment the frequency is 88 Hz. A signal generator,operating at the specified frequency, applies a signal current to thesignal electrode.

In the case of alternating current the conductance is identical to thereal part of the complex admittance, and therefore not necessarilyidentical with the inverse value of the resistance. An advantage ofusing alternating current instead of direct current in conductancemeasurement is that by this means one avoids the invidious effect on themeasurements of the skin's electrical polarizing properties.

The resulting current through the measuring electrode is conveyed to ameasurement converter 4. This comprises a current to voltage converter,which in a preferred embodiment is a transresistance amplifier, but inits simplest form may be a resistance, which converts the current fromthe measuring electrode to a voltage.

The measurement converter further comprises a decomposition circuit,preferably in the form of a synchronous rectifier, which decomposes thecomplex admittance in a real part (the conductance) and an imaginarypart (the susceptance). However, it is sufficient if the decompositioncircuit only comprises means for deriving the conductance. Thesynchronous rectifier multiplies the measured voltage with the voltagefrom the signal generator. The two signals are in-phase. Aftermultiplication, the result is according to the cosine (2u) equation,where the result is a DC component and one component at 2u frequency. Inthe preferred embodiment, this becomes 176 Hz. In the preferredembodiment, this synchronous rectifier is realized as an analog circuitwith the required accuracy.

The measurement converter 4 may also comprise amplifier and filtercircuits. In the preferred embodiment the measurement converter containslow-pass filters, both at the input and at the output. The object of theinput low-pass filter is to attenuate high-frequency noise, for instancecoming from other medical equipments, and also to serve as anti-aliasingfilter to prevent high frequency components from being received bysubsequent circuits for time discretization. The output low-pass filtershall attenuate the 2u components that result from the multiplicationoperation in the synchronous rectifier so that only the signal near DCis used for further processing.

By means of the choice of components and design details, moreover, themeasurement converter is designed with a view to obtaining highsensitivity and a low noise level.

The control unit 5 comprises a time discretization unit 51 for timediscretization of the signal from the measurement converter. The timediscretization takes place at a sampling rate, which may advantageouslybe in the order of 20 to 200 samplings per second. The control unitfurther comprises an analog-digital converter 52, which convertsmeasurement data to digital form. The choice of circuits for timediscretization and analog-digital conversion implies technical decisionssuitable for a person skilled in the art. In the preferred embodiment,time discretization is done in an integrated circuit, which combinesoversampling, filtering and discretization.

The control unit may advantageously comprise additional analog andpossibly also digital inputs (not illustrated), in addition to the inputfrom the measurement converter 4. In this case the control unit 5 caneither be equipped with a plurality of analog-digital converters 52, orit can employ various multiplexing techniques well-known to thoseskilled in the art in order to increase the number of analog inputs.These additional analog inputs may, for example, be arranged foradditional electrodermal measurements, or for other physiologicalmeasurements which may advantageously be performed simultaneously orparallel with the electrodermal measurement, such as temperature, pulse,ECG, respiratory measurements, oxygen saturation measurements in theblood, or EEG (bispectral index).

The control unit 5 also comprises a processing unit 53 for processingthe digitized measurement data, storage means in the form of at leastone store for storing data and programs, illustrated as a non-volatilememory 54 and a random access memory 55. The control unit 5 furthercomprises an interface circuit 61, which provides output signals 71, 72.Preferably, the control unit 5 further comprises a further interfacecircuit 81, which is further connected to display unit 8. The controlunit 5 may also advantageously comprise a communication port 56 fordigital communication with an external unit, such as a personal computer10. Such communication is well-suited for loading or altering theprogram which is kept stored in the memory 54, 55 in the control unit,or for adding or altering other data which are kept stored in the memory54, 55 in the control unit. Such communication is also well suited forread-out of data from the memory 54, 55 in the apparatus, thus enablingthem to be transferred to the external computer 10 for further,subsequent analysis or storage. A communication port 56 in the controlunit will be advantageously designed in accordance with requirements forequipment safety for patients, as described in more detail below.

In a preferred embodiment the non-volatile memory 54 comprises aread-only storage in the form of programmable ROM circuits, containingat least a program code and permanent data, and the random access memory55 comprises a read and write storage in the form of RAM circuits, forstorage of measurement data and other provisional data.

The control unit 5 also comprises an oscillator (not shown), whichdelivers a clock signal for controlling the processing unit 53. Theprocessing unit 53 also contains timing means (not shown) in order toprovide an expression of the current time, for use in the analysis ofthe measurements. Such timing means are well-known to those skilled inthe art, and are often included in micro controllers or processorsystems which the skilled person will find suitable for use with thepresent invention.

The control unit 5 may be realized as a microprocessor-based unit withconnected input, output, memory and other peripheral circuits, or it maybe realized as a micro controller unit where some or all of theconnected circuits are integrated. The time discretization unit 51and/or analog-digital converter 52 may also be included in such a unit.The choice of a suitable form of control unit 5 involves decisions,which are suitable for a person skilled in the art.

An alternative solution is to realize the control unit as a digitalsignal processor (DSP).

The control unit 5 is arranged to read time-discrete and quantizedmeasurements for the skin conductance from the measurement converter 4,preferably by means of an executable program code, which is stored inthe non-volatile memory 54 and which is executed by the processing unit53. It is further arranged to enable measurements to be stored in theread and write memory 55. By means of the program code, the control unit5 is further arranged to analyze the measurements in real time, i.e.simultaneously or parallel with the performance of the measurements. Inthis context, simultaneously or parallel should be understood to meansimultaneously or parallel for practical purposes, viewed in connectionwith the time constants which are in the nature of the measurements.This means that input, storage and analysis can be undertaken inseparate time intervals, but in this case these time intervals, and thetime between them, are so short that the individual actions appear tooccur concurrently.

The control unit 5 is further arranged to identify an average value forthe discrete, quantized measuring signal during a time interval, bymeans of a program code portion which is stored in the non-volatilememory 54 and which is executed by the processing unit 53.

The control unit 5 is further arranged to identify the fluctuations inthe time-discrete, quantized measuring signal, by means of a programcode portion which is stored in the non-volatile memory 54 and which isexecuted by the processing unit 53.

The control unit 5 is further arranged to count or calculate the numberof fluctuation peaks in the time-discrete, quantized measuring signalduring a time interval, by means of a program code portion which isstored in the non-volatile memory 54 and which is executed by theprocessing unit 53.

The processing unit 53, the memories 54, 55, the analog/digitalconverter 52, the communication port 56, the interface circuit 81 andthe interface circuit 61 are all connected to a bus unit 59. Thedetailed construction of such bus architecture for the design of amicroprocessor-based instrument is regarded as well-known for a personskilled in the art.

The interface circuit 61 is a digital port circuit, which derivesdigital output signals 71, 72 from the processing unit 53 via the busunit 59 when the interface circuit 61 is addressed by the program codeexecuted by the processing unit 53.

The first digital output signal 71 indicates that the analysis of theskin conductance measurement has detected that a state ofpain/discomfort has occurred in the patient. The second output signal 72indicates that a state of awakening has occurred in the patient.

In a special application of the invention the warning signals 71, 72 oranother signal derived from the processing means in the analysis of theskin conductance measurements may be used to control an automaticadministration of a medication to the patient. Particularly, theadministration of an analgesic medication may be controlled by the firstsignal 71 indicating pain/discomfort, and the administration of asleep-inducing medication or hypnotic may be controlled by the secondsignal 72 indicating awakening. Each of the signals may be used, forexample, to control a device for intravenous supply of medication. Inthis case the invention will form part of a feedback loop for control ofthe activity in the patient's autonomous nervous system.

In a preferred embodiment the display means 8 consist of a screen forgraphic visualization of the conductance signal, and a digital displayfor displaying the frequency and amplitude of the measured signalfluctuations. The display units are preferably of a type whose powerconsumption is low, such as an LCD screen and LCD display. The displaymeans may be separate or integrated in one and the same unit.

The apparatus further comprises a power supply unit 9 for supplyingoperating power to the various parts of the apparatus. The power supplymay be a battery or a mains supply of a known type.

The apparatus may advantageously be adapted to suit the requirementsregarding hospital equipment, which ensures patient safety. Such safetyrequirements are relatively easy to fulfill if the apparatus isbattery-operated. If, on the other hand, the apparatus is mainsoperated, the power supply shall meet special requirements, orrequirements are made regarding a galvanic partition between parts ofthe apparatus (for example, battery operated), which are safe for thepatient and parts of the apparatus, which are unsafe for the patient. Ifthe apparatus has to be connected to external equipment, which is mainsoperated and unsafe for the patient, the connection between theapparatus, which is safe for the patient and the unsafe externalequipment requires to be galvanically separated. Galvanic separation ofthis kind can advantageously be achieved by means of an opticalpartition. Safety requirements for equipment close to the patient andsolutions for fulfilling such requirements in an apparatus like that inthe present invention are well-known to those skilled in the art.

FIG. 2 illustrates a flow chart for a method for controlling a warningsignal in an apparatus for monitoring the autonomous nervous system of asedated patient, and especially for detecting pain/discomfort andawakening.

The method starts at reference 31.

The first two process steps 32 and 33 are initial steps, establishinginitial values for use in the remaining, repeated process steps.

In the first step 32, a skin conductance signal or EDR (electrodermalresponse) signal is measured, time-quantized and converted to digitalform using the equipment described with reference to FIG. 1. An initialtime-series of a certain duration, typically a period of 20 seconds,containing skin conductance data, is acquired during this step. With asampling rate of 20-200 samples per second, the time-series may contain400-4000 samples.

This time-series is then analyzed. In step 33, an average conductancelevel or basal level through the initial time-series is calculated. Thisinitial average conductance value is stored and used as the first“previous value” during the first execution of the comparison step 40below.

In step 35, a skin conductance signal is again measured, time-quantizedand converted to digital form using the equipment described withreference to FIG. 1. A time-series of a certain duration, typically aperiod of 20 seconds, containing skin conductance data, is acquiredduring this step.

This time-series is then analyzed. In step 36, an average conductancelevel or basal level through the current time-series is calculated. Thisinitial average conductance value is stored and used as the currentconductance average value during the execution of the comparison step 40below.

In step 37, the number of fluctuation peaks in the conductance signalthrough the current time-series is calculated. This is performed bydetecting local peaks or local maximum values and/or by detecting localvalleys or local minimum values. Although the following detaileddescription refers to detecting peaks, the skilled person will realizethat detecting valleys may be performed in an analogous way.

The existence of a peak is established if the derivative of the signalchanges sign through a small period in the interval. The derivative ofthe signal is calculated as the difference between two subsequent samplevalues. In addition, it is possible to use a simple digital filter thatneeds to see two or more subsequent sign changes before the sign changeis accepted.

In the calculation step 37 it may be necessary to establish additionalcriteria for when a peak should be considered as valid. In theirsimplest form such criteria may be based on the fact that the signal, inaddition to the sign change of the derivative, has to exceed an absolutelimit in order to be able to be considered a peak. A recommended, suchlimit value for the conductance is 0.02 μS.

Alternatively or in addition, it is an advantage to base the criteria onthe fact that the signal actually has formed a peak that has lasted acertain time. The criteria may also be based on the fact that theincrease in the skin conductance signal value as a function of time mustremain below a certain limit, typically 20 μS/s, if the maximum value isto be considered valid.

Another possible condition for establishing a valid peak, is that theabsolute value of the change in the conductance signal from a local peakto the following local valley exceeds a predetermined value, such as0.02 μS.

Also, a maximum value appearing at the border of the interval, i.e. thestarting point or ending point of the interval, should preferably not beregarded as a valid peak.

The object is thereby achieved that artifacts, which can occur in errorsituations such as, e.g., electrodes working loose from the skin, orother sources of noise or disturbances, does not lead to the erroneouslydetection of peaks.

The number of peaks calculated in step 37 is stored and used as thecurrent number of peaks during the execution of the comparison step 38below.

The conductance average calculating step 36 and the peak counting step37 could alternatively be performed in reverse order, or concurrently,if desired.

The purpose of the following steps 38-42 is to realize the followingfunctions:

If the number of peaks is above a certain limit, but the averageconductance level is unchanged, then pain/discomfort is detected, outputsignal 71 is activated and output signal 72, if previously activated, isreset.

If the number of peaks is above said limit and the average conductancelevel is increasing, then a state of awakening is detected, outputsignal 72 is activated and output signal 71, if previously activated, isreset.

If neither of the above conditions is achieved, then output signal 71 or72, if previously activated, is reset.

In the comparison step 38, the current calculated number of peaks iscompared with a preset limit value. The Applicant's tests have shownthat a suitable limit value is 0.1 peaks per second, i.e. 2 peaks per 20seconds. Other values could possible be determined from clinical tests,in order to further optimize the performance and reliability of theoutput indications.

If the current number of peaks is equal to or higher than the presetlimit value (output denoted Y), the process continues to the decisionstep 40. If on the other hand the current number of peaks is smallerthan the preset limit value (output denoted N), the process is continuedat step 39.

In step 39, both output signals 71 or 72 are reset, if any of them werepreviously activated.

In comparison step 40, the current average conductance value is comparedwith the previous average conductance value. If the current averageconductance value is smaller than or equal to the previous averageconductance value (output denoted N), the state of pain/discomfortshould be indicated, and the process continues to step 41. If on theother hand the current average conductance value is larger (outputdenoted Y), the state of awakening should be indicated, and the processcontinues to step 42.

In step 41, the state of pain/discomfort is indicated. The processingunit 53 activates the first output signal 71, indicating apain/discomfort state, via the interface circuit 61, and apain/discomfort message is indicated on the display unit 8 by the use ofthe interface circuit 81. If the second output signal 72 is previouslyactivated, it is reset. The process is then continued at the updatingstep 43.

In step 42, the state of awakening is indicated. The processing unit 53activates the second output signal 72, indicating an awakening state,via the interface circuit 61, and an awakening message is indicated onthe display unit 8 by the use of the interface circuit 81. If the firstoutput signal 71 is previously activated, it is reset. The process isthen continued at the updating step 43.

In the updating step 43, the current average conductance value is storedas the previous average conductance value. The process is then repeatedfrom step 35.

The process may be interrupted or terminated by an operating device (notshown) or by a command input from the communication port 56.

A first improvement to the method illustrated in FIG. 2 will bedescribed in the following:

In the comparison step 38 in FIG. 2, the current calculated number ofpeaks is compared with a preset limit value. Even more reliable resultsmay be achieved for the pain/discomfort and awakening indications ifthis comparison is also dependent on the condition that the currentnumber of peaks is larger than the previous number of peaks.

In order to perform this extended comparison, an additional step 34should be performed subsequent to step 33, wherein the number offluctuation peaks in the conductance signal through the initial periodis calculated. This calculation is performed in the same way asdescribed with reference to step 37. The initial number of peaks isstored and used as the “previous number of peaks” in the first executionof the extended comparison step 38.

Further, the comparison step 38 should be modified. In the modifiedcomparison step 38, the current number of peaks is compared with thepreset limit value and with the previous number of peaks. If the currentnumber of peaks is larger than both the limit value and the previousnumber of peaks, the process continues to the comparison step 40. If onthe other hand the number of peaks is equal to or less than the limitvalue or the previous number of peaks, or both, the process continues tostep 39.

The updating step 43 should also be modified. In the modified updatingstep 43, the current number of peaks is stored as the previous number ofpeaks. In addition, the current average conductance value is stored asthe previous average conductance value.

A second improvement to the embodiment illustrated in FIG. 2 will bedescribed in the following:

In the embodiment in FIG. 2, a time-series is first acquired andsubsequently analyzed. As an advantageous alternative, data acquisitionand analysis are performed as separate, independent processes,concurrently executed by the processing unit 53.

A data acquisition process is then performed, which virtuallycontinuously updates a portion of the memory 55 with the latest e.g. 20seconds of skin conductance signal values.

An analysis process is initiated e.g. every 1 second. This process willanalyze the latest e.g. 20 seconds of skin conductance data, acquired bythe concurrently executed data acquisition process. All the processsteps 35-43 are performed by the analysis process, while the initialprocess steps 32 and 33 are performed in advance, as initial steps.

This solution leads to an even faster and more reliable indication ofpain/discomfort and awakening, compared to the simpler method describedwith reference to FIG. 2.

FIGS. 3 a-c are three graphs, each illustrating a time-series of a skinconductance measurement signal (vertically) vs. time (horizontally) of asedated patient, which is exposed to neither awakening stimuli norpain/discomfort stimuli,

FIG. 3 a is a graph illustrating a skin conductance signal which isessentially steady.

A first time interval of about 20 seconds is indicated by 301 a, and asecond time interval of about 20 seconds is indicated by 302 a.

Assume that the method according to the embodiment described withreference to FIG. 2 is applied to this signal, with the initialtime-series corresponding to the time interval indicated by 301 a, andthe next time-series corresponding to the time interval indicated by 302a. The preset limit value is 2 peaks per 20 seconds. In the second timeinterval 302 a the number of peaks will be calculated as zero. Then thecurrent number of peaks will be less than the preset limit value.Consequently, the process continues to step 39, i.e. the pain/discomfortstate signal and the awakening signal are both reset. The monitoringprocess will then be repeated, based on the time interval 302 a as theprevious time interval and a subsequent time interval (not illustrated)as the current time interval.

FIG. 3 b is a graph illustrating a time-series of skin conductancemeasurements of a patient whose skin conductance is steadily decreasing.

A first time interval of about 20 seconds is indicated by 301 b, and asecond time interval of about 20 seconds is indicated by 302 b.

Assume now that the method according to the embodiment described withreference to FIG. 2 is applied to this signal, with the initialtime-series corresponding to the time interval indicated by 301 b, andthe next time-series corresponding to the time interval indicated by 302b. In the second time interval 302 b the number of peaks will becalculated as 0. Then the number of peaks is recognized as below thepreset limit of 2 per 20 seconds. Then the current number of peaks willbe less than the preset limit value. Consequently, the process continuesto step 39, i.e. the pain/discomfort state signal and the awakeningsignal are both reset. The monitoring process will then be repeated,based on the time interval 302 b as the previous time interval and asubsequent time interval (not illustrated) as the current time interval.

FIG. 3 c is a graph illustrating a time-series of skin conductancemeasurements of a patient whose skin conductance is steadily increasing.

A first time interval of about 20 seconds is indicated by 301 c, and asecond time interval of about 20 seconds is indicated by 302 c.

Assume now that the method according to the embodiment described withreference to FIG. 2 is applied to this signal, with the initialtime-series corresponding to the time interval indicated by 301 c, andthe next time-series corresponding to the time interval indicated by 302c.

The apparent fluctuations in the signal are due to noise with arelatively low magnitude (less than 0.02 μS). Provided that step 37 isimplemented with the additional condition that the absolute value of thechange in the conductance signal from a local peak to the followinglocal valley should exceed the predetermined value 0.02 μS in order toconsider a peak as valid, the apparent peaks will not be considered asvalid peaks.

In the second time interval 302 c the number of peaks will thus becalculated as 0. Then the number of peaks is recognized as below thepreset limit value of 2 per 20 seconds. Then the current number of peakswill be less than the preset limit value. Consequently, the processcontinues to step 39, i.e. the pain/discomfort state signal and theawakening signal are both reset. The monitoring process will then berepeated.

FIG. 4 is a graph illustrating a time-series of skin conductancemeasurements of a sedated patient, which is exposed to pain/discomfortstimuli.

A first time interval of about 20 seconds is indicated by 401, and asecond time interval of about 20 seconds is indicated by 402. Twosubsequent peaks out of several peaks in the skin conductance signal areindicated by 403 and 404.

Assume now that the method according to the embodiment described withreference to FIG. 2 is applied to this signal, with the initialtime-series corresponding to the time interval indicated by 401, and thenext time-series corresponding to the time interval indicated by 402. Inthe second time interval 402 the number of peaks will be calculated as10. Then the number of peaks is recognized as equal to or above thepreset limit. Consequently, the comparison step 40 will be executed.

Further, the average skin conductance value through the first timeinterval 401 will be calculated as about 8.3 microsiemens, and theaverage skin conductance value through the second time interval 402 willbe calculated as about 8.2 microsiemens.

Then no increase in the average conductance value will be recognized inthe comparison step 40. Thus, step 41 is entered, which means that thefirst output signal 71 is activated, and a state of pain/discomfort inthe patient is indicated.

Then the process continues to step 43, i.e. the monitoring process isrepeated, based on the time interval 402 as the previous time intervaland a subsequent time interval (not illustrated) as the current timeinterval.

FIG. 5 is a graph illustrating a time-series of skin conductancemeasurements of a sedated patient, which is exposed to awakeningstimuli.

A first time interval of about 20 seconds is indicated by 501, and asecond time interval of about 20 seconds is indicated by 502. Two peaksin the skin conductance signals are indicated by 503 and 504.

Assume now that the method according to the embodiment described withreference to FIG. 2 is applied to this signal, with the initialtime-series corresponding to the first time interval indicated by 501,and the next time-series corresponding to the second time intervalindicated by 502. In the second time interval 502 the number of peakswill be calculated as 2. In the comparison step 38, the number of peaksis recognized as equal to or above the preset limit. Consequently, thecomparison step 40 will be executed.

The average skin conductance value through the first time interval 501will be calculated as about 2.4 microsiemens, and the average skinconductance value through the second time interval 502 will becalculated as about 3 microsiemens.

Consequently, an increase in the average conductance value is recognizedin the comparison step 40. Thus, step 42 is executed and the secondoutput signal 72 is activated, which means that a state of awakening inthe patient is indicated. Then the process continues to step 43, i.e.the monitoring process is repeated, based on the time interval 502 asthe previous time interval and a subsequent time interval (notillustrated) as the current time interval.

The above description and drawings present a specific embodiment of theinvention, with the addition of some alternatives. For a person skilledin the art, however, it will be obvious that other, alternativeembodiments exist which are within the scope of the present invention.For instance, the skin conductance signal may be measured using a DCmethod instead of the specifically described AC method. The use of skinresistance instead of skin conductance as the measurement signal will ofcourse lead to equivalent results, if the inverse nature of thesevariables is taken into account. Although the detection of peaks arespecified in the detailed description, the skilled person will realizethat the same result will appear if valleys or minimum points aredetected in a similar way.

Moreover, when a patient is exposed to induction of anesthesia, thenumber of peaks will decrease together with a decrease in averageconductance level.

The inventive concept is thus not limited to the exemplary embodimentsdescribed above. Rather, the scope of the invention is set forth in thefollowing patent claims.

1-13. (canceled)
 14. Method for monitoring the autonomous nervous systemof a sedated patient, comprising providing a skin conductance signalmeasured at an area of the patient's skin, calculating characteristicsof said skin conductance signal, establishing a first output signalindicating the state of pain/discomfort in the patient, based on saidcharacteristics of said skin conductance signal, wherein saidcharacteristics comprise the average value of the skin conductancesignal through a first time interval and the average value of the skinconductance signal through a second time interval, and the number offluctuation peaks or valleys of the skin conductance signal through saidsecond time interval, and establishing a second output signal indicatingthe state of awakening in the patient, based on said characteristics ofsaid skin conductance signal, said establishing step including comparingthe number of fluctuation peaks or valleys through said second timeinterval with a limit value, and if the number of peaks or valleys ishigher than the limit value, comparing the average conductance valuethrough the second interval with the average conductance value throughthe first interval, and if the second average conductance value is thelarger, establishing the second output signal as indicating the state ofawakening in the patient.
 15. Method according to claim 14, wherein saidstep of establishing the first output signal comprises comparing thenumber of fluctuation peaks or valleys through said second time intervalwith a limit value, and if the number of peaks or valleys is higher thanthe limit value, comparing the average conductance value through thesecond interval with the average conductance value through the firstinterval, and if the second average conductance value is not the larger,establishing the first output signal as indicating the state ofpain/discomfort in the patient.
 16. Method according to one of theclaims 14-15, wherein the first and second intervals have equalduration, e.g. 20 seconds.
 17. Method according to claim 16, wherein thestart point of the second interval is 0.5 to 5 seconds subsequent to thestart point of the first interval.
 18. Method according to claim 17,wherein the start point of the second interval is approximately 1 secondsubsequent to the start point of the first interval.
 19. Methodaccording to one of the claims 14-15, wherein the start point of thesecond interval essentially coincides with the end point of the firstinterval.
 20. Apparatus for monitoring the autonomous nervous system ofa sedated patient, comprising measurement equipment, providing a skinconductance signal measured at an area of the patient's skin, a datastorage for storing the measured signal values at discrete points oftime, a control unit arranged for calculating characteristics of saidskin conductance signal, establishing a first output signal indicatingthe state of pain/discomfort in the patient, based on saidcharacteristics of said skin conductance signal, said characteristicscomprising the average value of the skin conductance signal through afirst time interval and the average value of the skin conductance signalthrough a second time interval, and the number of fluctuation peaks orvalleys of the skin conductance signal through said second timeinterval, and establishing a second output signal indicating the stateof awakening in the patient based on said characteristics of said skinconductance signal, by performing the steps of comparing the number offluctuation peaks or valleys through said second time interval with alimit value, and if the number of peaks or valleys is higher than thelimit value, comparing the average conductance value through the secondinterval with the average conductance value through the first interval,and if the second average conductance value is the larger, establishingthe second output signal as indicating the state of awakening in thepatient.
 21. Apparatus according to claim 20, wherein the control unitis arranged to establish the first output signal by performing the stepsof comparing the number of fluctuation peaks or valleys through saidsecond time interval with a limit value, and if the number of peaks orvalleys is higher than the limit value, comparing the averageconductance value through the second interval with the averageconductance value through the first interval, and if the second averageconductance value is not the larger, establishing the first outputsignal as indicating the state of pain/discomfort in the patient. 22.Apparatus according to one of the claims 20-21, wherein the first andsecond intervals have equal duration, e.g. 20 seconds.
 23. Apparatusaccording to claim 22, wherein the start point of the second interval is0.5 to 5 seconds subsequent to the start point of the first interval.24. Apparatus according to claim 23, wherein the start point of thesecond interval is approximately 1 second subsequent to the start pointof the first interval.
 25. Apparatus according to claim 22, wherein thestart point of the second interval essentially coincides with the endpoint of the first interval.
 26. Apparatus according to one of theclaims 20-21, further comprising a device for supply of medication tothe patient, controlled by the first output signal or the second outputsignal.
 27. Apparatus according to one of the claims 20-21, furthercomprising a device for supply of medication to the patient, controlledby the first output signal and the second output signal.
 28. Methodaccording to one of the claims 14-15, further comprising the step ofsupplying medication to the patient, controlled by the first outputsignal or the second output signal.
 29. Method according to one of theclaims 14-15, further comprising the step of supplying medication to thepatient, controlled by the first output signal and the second outputsignal.